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markosheehan

Member
Jun 6, 2016
136
can someone help me with the 1989 question.

WIN_20171006_19_04_14_Pro.jpg

i think part i is k2(c03)
for ii the gas given of is co2. 1 one the precipitates given off was caco3. i am not sure what the other was. is it supposed to be a reaction between mgso4 +k2co3?
 

Klaas van Aarsen

MHB Seeker
Staff member
Mar 5, 2012
9,491
can someone help me with the 1989 question.

i think part i is k2(c03)
for ii the gas given of is co2. 1 one the precipitates given off was caco3. i am not sure what the other was. is it supposed to be a reaction between mgso4 +k2co3?
Hi markosheehan ,

Indeed, the lilac color indicates we have K.
And the white precipitate of the colorless gas in lime water suggests CO2.
So X is probably $\ce{K2CO3}$.

And yes, when combining a solution of $\ce{K2CO3}$ with a solution of $\ce{MgSO4}$, we have:
$$\ce{2K+ + CO3^{2-} + Mg^{2+} + SO4^{2-}}$$
Since K is very soluble, the precipitate must be from Mg.
And since we can assume that it won't be the original magnesium sulfate, which was dissolved after all, that leaves us with only one choice.
It should be $\ce{MgCO3}$, which is white, confirming our analysis.
 

markosheehan

Member
Jun 6, 2016
136
thanks for your help

how did you know "Since K is very soluble, the precipitate must be from Mg."

is it just a general rule you just learn off that potassium is more soluble than mg. just because k is more soluble than mg how did you know mg would join with the (co3)^2-

thanks
 

Klaas van Aarsen

MHB Seeker
Staff member
Mar 5, 2012
9,491
thanks for your help

how did you know "Since K is very soluble, the precipitate must be from Mg."

is it just a general rule you just learn off that potassium is more soluble than mg. just because k is more soluble than mg how did you know mg would join with the (co3)^2-

thanks
Potassium is in group 1, meaning it has a single electron in its outer shell that it is eager to release, becoming an ion that is dissolved in water.
This holds true for all metals in group 1 (Li, K, Na).
So we can expect Mg (group 2) to be less soluble than K.
This is a general rule -- chemical intuition if you will.

That means that Mg must join with either CO3 or with SO4.
Since it was already dissolved with SO4, the obvious candidate is CO3.
 

markosheehan

Member
Jun 6, 2016
136
thanks